Optimisation of a MSSQL Query - group by multiple columns - sql-server

Question

Hey guys i could need some advice, i've the following 2 tables
Table Model:
+----------------+---------------+-------------+------------------------+-------------+--------+-----------------+------------------+------------------+------------------------------+------------------------------+----------------------+----------------------+------------------+
| DLTCountryCode | SupplierID | ModelNumber | ModelDescription | Brand | Fedas | MeasurementUnit | MinModelNetPrice | MaxModelNetPrice | MinModelSuggestedRetailPrice | MaxModelSuggestedRetailPrice | MinModelInsteadPrice | MaxModelInsteadPrice | PictureAvailable |
+----------------+---------------+-------------+------------------------+-------------+--------+-----------------+------------------+------------------+------------------------------+------------------------------+----------------------+----------------------+------------------+
| AT | 9120048150008 | 2012266 | xxx | Brand | 115946 | STK | 6.05 | 6.05 | 10.95 | 10.95 | 0 | 0 | 1 |
+----------------+---------------+-------------+------------------------+-------------+--------+-----------------+------------------+------------------+------------------------------+------------------------------+----------------------+----------------------+------------------+
Table ModelColorSizeInventory:
+----------------+---------------+-------------+-----------+------+---------------+----------+-------------------------+
| DLTCountryCode | SupplierID | ModelNumber | ColorCode | Size | ItemNumber | Quantity | InventoryDateTime |
+----------------+---------------+-------------+-----------+------+---------------+----------+-------------------------+
| AT | 9120048150008 | 2012266 | 801 | L | 9008601584968 | 0 | 2017-09-29 11:16:02.347 |
| AT | 9120048150008 | 2012266 | 801 | M | 9008601584951 | 0 | 2017-09-29 11:16:02.347 |
| AT | 9120048150008 | 2012266 | 801 | S | 9008601584944 | 2 | 2017-09-29 11:16:02.347 |
| AT | 9120048150008 | 2012266 | 801 | XL | 9008601584975 | 4 | 2017-09-29 11:16:02.347 |
| AT | 9120048150008 | 2012266 | 801 | XXL | 9008601584982 | 6 | 2017-09-29 11:16:02.347 |
+----------------+---------------+-------------+-----------+------+---------------+----------+-------------------------+
And the following Query:
SELECT dccdm.*, SUM(dccdmcsi.[Quantity]) AS QuantityModel
FROM "Model" AS "dccdm"
LEFT JOIN ModelColorSizeInventory AS dccdmcsi ON dccdm.[ModelNumber] = dccdmcsi.[ModelNumber]
WHERE (
dccdm.ModelNumber IN('2012266')
)
AND dccdmcsi.[Quantity] >0
AND dccdm.[DLTCountryCode]='AT'
GROUP BY dccdm.[DLTCountryCode],dccdm.[SupplierID],dccdm.[ModelNumber],dccdm.[ModelDescription],dccdm.[Brand],dccdm.[Fedas],dccdm.[MeasurementUnit],dccdm.[MinModelNetPrice],dccdm.[MaxModelNetPrice],dccdm.[MinModelSuggestedRetailPrice],dccdm.[MaxModelSuggestedRetailPrice],dccdm.[MinModelInsteadPrice],dccdm.[MaxModelInsteadPrice],dccdm.[PictureAvailable]
This Query works as expected, i'm joining ModelColorSizeInventory to find out the sum of all variants with quantities
One thing that bothers me is the group by part, because if i skip the group by statement i'm getting the following error:
Msg 8120, Column 'DLTCountryCode' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
Since i'm not that familiar with MSSQL i ask the following question:
How can i write down this query without using this complex GROUP BY clause
The reason behind this question is that writing down multiple columns in a group by statement feels wrong, in queries like these ... ;)

Answer 1

You could use "subview" for grouped amounts, or simply use subquery in join, like:
Select dccdm.*, Isnull(dccdmcsi.SumQuantity,0)
FROM dbo.[Model] dccdm LEFT JOIN
(Select ModelNumber, SUM([Quantity]) as SumQuantity from dbo.ModelColorSizeInventory GROUP BY ModelNumber) dccdmcsi
ON dccdm.ModelNumber=dccdmcsi.ModelNumber